A large number of chemicals do not degrade readily, rather, they end up bound to fine particles and remain as a threat for years in bottom sediments, or concentrated in plants or wildlife.

Dosis facit venenum. In the 16th century, alchemist Paracelsus already knew that the dose makes the difference between poison and remedy, meaning that all substances (including water) can be poisonous or at least harmful in high doses. Centuries later, we know that it is not only the dose (or concentration), but duration of exposure, bioaccessibility, bioavailability and many, many other factors which contribute to relative toxicity of a substance. But how do we set acceptable levels of substances taht we are certain would cause no harm, in terms of lethal or sub-lethal effect, to humans and all other living organisms?

Ecotoxicology as a discipline is concerned with the study of toxic effects, caused by natural or synthetic pollutants, and aims to quantify and predict the magnitude of the stress on natural populations, communities or ecosystems so that the most efficient and effective action to prevent any detrimental effect can be identified, or in ecosystems that are already impacted by pollution, to identify the best course of action to restore ecosystem services and functions.

Understanding long-term effects. Focusing on an individual substance, it seems relatively simple – using various single-substance ecotoxicological tests and by the application of so-called safety factors, we can set the acceptable daily intake, effluent limits and even water quality standards. But by doing so, we are not even halfway through – because in real aquatic ecosystems, natural populations are exposed to a myriad of different chemicals simultaneously: as they are being discharged from direct sources, accidently spilled into water or end up in aquatic ecosystems via run-off or atmospheric precipitation. In addition, through chemical and (micro) biological degradation, original compounds might be transformed into even more toxic by-products.

Unfortunately, the problem of hazardous substances in aquatic ecosystems comes into the limelight only in the case of big accidental spills, such as the wellknown Baya Mare cyanide accident, or through oil spills. Though occurring rarely, high concentrations of toxic substances cause spectacular fish kills, which are reported in mass media, but long-term chronic effects and sub-lethal toxicity – which might cause changes in behaviour, inhibition of growth, reproduction, or alterations of functional metabolic processes (photosynthesis, respiration) or even have terathogenic effects – do not generate as much attention as they are not apparent; they remain hidden behind the curtains of our general ignorance of ecosystem complexity and functions.

Narrow monitoring programmes. In spite of the enormous number of possible contaminants in the environment, risk assessment of toxic pollution in aquatic ecosystems is still based on few pre-selected and regularly monitored target compounds. The EU Water Framework Directive (WFD) has not changed the concept of toxic pollution monitoring and risk assessment. On the contrary, traditional, conservative monitoring programmes which rely on monitoring of substances rather than effects, underpinned with the list of 33 compounds selected as priority pollutants by the European Commission, remain the accepted and widely used concepts all over Europe, including the Danube River Basin.

The current EU list of priority pollutants is so short that official monitoring programmes are rather conservative. They say little to nothing about bioavailability, toxicity and, hence, risk to ecosystems from hazardous substances; and they pay almost no attention to emerging and other substances beyond this list. The introduction of a regularly monitored basin-specific list of pollutants could change this picture. The WFD classifies the quality status of aquatic ecosystems based on traditional hydromorphological, physico-chemical, biological parameters and priority pollutant (PP) concentrations. While this procedure allows for a rough quality assessment, a reliable diagnosis and prediction of toxic impacts on aquatic ecosystems and an efficient mitigation of toxic risks requires identifying the key toxic pollutants and understanding the cause–effect relationships between chemical pollution and biodiversity decline.

The long-term chronic effects of toxicity include changes in behaviour, inhibition of growth, reproduction, or alterations of functional metabolic processes (photosynthesis, respiration) or even birth effects.

The role of sediments. The EU Priority Substances Directive, adopted in 2008, aims to ensure a high level of protection against the risks of priority substances and other pollutants to the aquatic environment. Since approximately 80% of the listed priority substances are retained by sediment and suspended particulate matter, it has been agreed that Member States may apply environmental quality standards for sediment or biota instead of those for water. The newly proposed guidelines should raise the monitoring requirements for controlling the environmental quality standards.

While these new regulations and guidelines could be seen as an ideal vehicle for addressing the important role of sediments in watershed quality, it is uncertain to what extent sediment quality will explicitly play a role in assessing ecological quality under the WFD – as it is not mandatory. The WFD directs Member States to monitor macrobenthic invertebrates and develop sediment quality standards, so there is clearly scope for consideration of sediment quality as an integral part of river basin management. However, a
preliminary overview of river basin management plans shows extreme inconsistency across Europe; sediment management issues were not an integral part of the Danube River Basin Management Plan nor does sediment quality assessment play an important role in assessing ecological/chemical status.

Better evaluation. To date, severe knowledge gaps impede the evaluation and mitigation of the causes for poor ecological status in many aquatic ecosystems. To address this, EU-funded projects such as Modelkey, AquaTerra, Liberation and NoMiracle, as well as related networks such as Norman, SedNet and RiskBase, have been recently launched to establish links between chemical quality of sediments and surface waters with measurable toxic effects.

Effect-based identification of key toxicants as well as analysis, modelling and assessment of bioavailability and food web accumulation are needed, as well as a better evaluation of monitoring data on contamination, toxicity and ecological quality at the basin level.

However, sound scientific concepts, models and decision support systems have to find their way to major stakeholders, water managers and even policy makers as their implementation would certainly contribute to the common European goal – achieving good ecological status.

As pollution by hazardous substances has been recognised as one of the significant water management issues within the Danube River Basin, some of the applied aspects of ecotoxicological research could bring practical benefits to water management.

For more information, please visit: www.iad.gs.

Ivana Teodorovic is the President of the International Association for Danube Research (IAD), and Associate Professor at the University of Novi Sad, Serbia, affiliated to the Faculty of Science, Laboratory of Ecotoxicology.